Take A Giant Step A New Dinosaur Exhibit Shatters Some Large Myths

Even to an adult who should know better, the dinosaur is downright frightening.

He looks out over the crowd below him, his head twisting at the end of a long, rubbery neck. He's searching for something. Lunch?

Suddenly his head drops, his eyes snap and he's gazing directly at you. He opens his mouth and roars. It's more of a grumpy complaint than a loud, blood- thirsty screech. Yet there is something chilling about it.

You know the 30-foot-long dinosaur isn't real. It's just a mechanical animation. Even if it were real, it only would be half-grown and strictly a vegetarian.

Dinosaurs are powerful and humbling. They are the stuff of legends and nightmares, the only real monsters that ever walked the Earth.

Perhaps they're so fascinating because it's hard to accept they were real - stars in a terrific chapter of Earth's history that happened so long ago it defies imagination.

This planet was theirs long before man came along, and they may have been its masters much longer than we ever will hold the deed. They ruled for nearly 160 million years and died out 65 million years ago. Man, in all his glory, has been around no more than 5 million years.

Be prepared to leave most of what you heard about prehistoric reptiles behind when you step into the new "Discovering Dinosaurs" exhibition at the Academy of Natural Sciences museum in Philadelphia.

Dinosaurs are far more alien than most people realize. They didn't stand upright, like humans, with their tails dragging on the ground. Instead, even those with two legs walked with their bodies parallel to the ground, their huge tails suspended behind them.

Dinosaurs were fast, smart, cared for their young, and may have traveled in herds and hunting packs. Some were much bigger,and others much smaller, than previously believed. One meat-eater was only the size of a rooster.

They also may have been very colorful - to attract mates, frighten enemies or hide from predators. Some scientists even believe they had feathers. Others say birds evolved from meat-eating dinosaurs which, unlike contemporary reptiles, might have been warm-blooded.

Dinosaurs have been brought to life in the $2.5-million exhibit, which opened in late January. And the exhibit has given new life to the museum. Although the academy doesn't have the biggest dinosaur display in the country, it claims "the most up-to-date exhibit in the world." More than a dozen skeletons and fleshed-out models of prehistoric creatures - including a nest of babies - fill the hall on the main floor.

By early summer, a skeleton of a young horned dinosaur, so rare it doesn't have a name yet, will complete the exhibit.

Tyrannosaurus Rex is the star of the show. He looms threateningly over those entering the hall, great jaws agape as though ready to snap up little children. He is the king of the dinosaurs, the largest known meat-eater.

Myths and legends about dragons and giants may be based upon skeletons of prehistoric animals that ancient cultures found but could not explain. In the museum, you can marvel at engravings of sea serpents, based on actual sightings, that bear a striking resemblance to fossils of marine reptiles found years later.

There were more than 500 different species of dinosaurs. But if it flew in the air or swam in the sea, it wasn't a dinosaur. A prehistoric reptile, yes, but not a dinosaur. Dinosaurs walked on the Earth. To make things more complicated, so did other reptiles that were not dinosaurs - like Dimetrodon, the creature with a sail-fin on its back.

The posture and hip structure ofdinosaurs set them apart from other reptiles. Their legs were directly under their bodies, not out to the side. Their tails were suspended to give them better balance. Fossilized dinosaur tracks almost never show evidence of dragging tails and skeletons of tails include bony tendons that kept them stretched out straight.

Studying dinosaurs involves "reasoned speculation" since a complete skeleton rarely is found. (Only an expert could tell which of the skeletons mounted in the museum are petrified bone, which are man-made casts of other bones and which contain a combination of both.) Since most dinosaurs decayed after they died, each fossil discovery is considered a "small miracle." Scientists even study coprolites, the petrified excrement of prehistoric animals.

The exhibit addresses some of at least 50 extinction theories about dinosaurs, including wild ones which suggest they were all poisoned by flowering plants.

One theory was that small mammals ate the dinosaur eggs. But then why did turtles, snakes, crocodiles and lizards - which also lay eggs - survive? And why did prehistoric marine reptiles also disappear? As the planet gradually cooled, why couldn't dinosaurs have adapted or why didn't at least those in the tropics survive?